Although optical isolators have conventionally been used for optical communications, due to advancements in the field of optical processors in recent years, optical processors have also come to require optical isolators. The wavelength required to be accommodated at that time is mainly the wavelength of 1064 nm of Nd:YAG lasers. Terbium gallium garnet (TGG: Tb3Ga5O12) single crystals have recently been developed and used practically as materials suitable for this wavelength (Non-Patent Document 1).
However, it is difficult to increase the size of TGG crystals due to the violent evaporation of gallium oxide, which is a raw material component thereof, and this has prevented the cost of these crystals from being able to be reduced. For this reason, the development of a material that has a larger Faraday rotation angle (Verdet constant) than TGG and can be produced at low cost has been sought (Non-Patent Document 1).
In order to solve the above-mentioned problem of TGG, development has proceeded on the growth of terbium aluminum-based garnet (TAG: Tb3Al5O12) single crystals. A known method used to grow TAG involves a production process using a melt growth method (LPE method) in which a substrate crystal is placed in a supercooled melt inside a crucible and crystals are grown on the surface thereof in the form of a film (Non-Patent Document 2).
In addition, research is also being conducted on terbium scandium aluminum garnet (TSAG: Tb3Sc2Al3O12) single crystals, and TSAG single crystals have been reported to be superior with respect to enlargement of crystal size (Patent Document 1).